10 JavaScript Interview Questions You Must Know (With Answers)

1. What is the difference between == and ===?

== performs type coercion before comparing. === compares both value and type without coercion. Always use === in production code to avoid unexpected type conversions.

2. Explain event delegation

Event delegation attaches a single event listener to a parent element instead of many listeners on children. Events bubble up the DOM, so the parent catches them. This is more memory-efficient and handles dynamically added elements automatically.

3. What is the difference between null and undefined?

undefined means a variable has been declared but not assigned. null is an intentional absence of value — a developer explicitly set it to null. Both are falsy, but typeof null is "object" (a historical JavaScript bug) while typeof undefined is "undefined".

4–10: More Critical Concepts

Other essential topics: the event loop and call stack, prototypal inheritance vs class-based, hoisting of var/let/const/functions, closure and lexical scoping, the this keyword in different contexts, shallow vs deep copy, and the difference between map/filter/reduce.